How Surfaces Make Fluorescence a Highly Sensitive Probe of Chirality

Fluorescence has long been considered a poor probe of chirality. In an interview with Spectroscopy, Garth Simpson, who is a Tarpo Professor of Chemistry at Purdue University, explained why that assumption breaks down at interfaces, and how his group has turned a simple geometric insight into a sensitive method for chiral characterization.

The conventional view rests on the physics of light–matter interactions. Chiral signals in isotropic media, such as subtle absorption differences and circularly polarized luminescence, arise from magnetic dipole interactions, which are far weaker than electric dipole interactions. Within the electric dipole approximation, a fluorophore’s behavior is described by two vectors, one for excitation and one for emission. Because two vectors define a plane, and a plane has mirror symmetry, these interactions alone cannot produce a chiral response.

A surface changes the picture. Placing molecules on a surface introduces a third vector and lowers the system’s symmetry from isotropic, or completely disordered, to uniaxial, which has a unique axis. Because surfaces are ubiquitous, this combination of surface ordering and intrinsic molecular chirality can produce a large chiral-specific response that is fully electric dipole allowed.

Simpson and his group predicted that exciting such a system with linearly polarized light would rotate the principal axis of the fluorescence emission clockwise or counterclockwise. To test it, they built an instrument and measured a rotation of about 4.5 degrees for a monolayer of naproxen. By comparison, circular dichroism (CD) signals are typically reported in millidegrees across roughly 1-cm path lengths, while the naproxen monolayer is about 5 to 10 Å thick. A back-of-the-envelope calculation suggested an enhancement of roughly 35 million-fold over chirality probed in an isotropic system.

Simpson described the effect as purely geometric and intuitive, with promise for high-sensitivity chiral analysis at interfaces. The group has also applied the principle to dark-field CD, which measures scattered rather than transmitted light. Because the mathematics of scattering mirrors that of fluorescence, the technique delivers large chirality sensitivities when the instrument isolates the scattered signal.

Further Reading

1. Purdue University, Garth J. Simpson. Purdue University, 2026. https://www.chem.purdue.edu/people/profile/gsimpson.html (accessed November 3, 2026).
2. Wetzel, W.; Spectroscopy Staff. EAS 2026 to Mark 65th Anniversary with Program Focused on AI and Data Integrity. Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/eas-2026-to-mark-65th-anniversary-with-program-focused-on-ai-and-data-integrity (accessed November 3, 2026).